The present invention is directed to a drive head assembly for a deep well, submersible, progressing cavity pump; and more particularly, to an assembly for slowing the counter rotation of the polished rod induced by recoil in the sucker rod, and further, to a clamp adapted to coaxially secure the polished rod to a main shaft of the drive head assembly in a manner that allows some relative vertical movement of the sucker rod and associated polished rod without the polished rod becoming disengaged from the main shaft.
Progressing cavity pumps (and other types of rotary pumps) are conventionally used as down-hole pumps in the oil production industry. The helical rotors of such progressing cavity pumps are driven by a rod string or sucker rod coaxially coupled to a polished rod, which is in turn driven by a drive assembly typically located above the surface. The drive assembly, powered by a motor, is adapted to rotatably drive the polished rod and associated sucker rod. Typically, the sucker rod is a metallic rod a few inches in diameter and thousands of feet in length, and is coaxially coupled between the polished rod and the helical rotor of the progressing cavity pump. Because a substantial torsional force is often required to start the helical rotor turning within the progressing cavity pump stator, the drive assembly will turn or twist the sucker rod many times (conventionally in the range of 40 to 50 times) before the rotor begins to turn within the stator. Torsional force used to twist the sucker rod the 40 or 50 times is stored in the elongated sucker rod until the motor is disengaged or shut off. Once the motor is disengaged or shut off, this stored energy (the extra twists in the sucker rod) will immediately begin to be released in the form of a rapid recoil of the sucker rod. Unless this recoil is controlled or slowed, the motor and associated drive belts can become damaged by the rapid counter rotation of the polished rod.
Known ways for controlling such recoil include a disc brake mechanism to slow the counter rotation of the polished rod, or an external hydraulic pump mechanism coupled to the drive assembly. However, these prior art methods have several known disadvantages. With a disc brake mechanism, a great deal of friction is created during recoil releasing a large amount of heat. This causes unnecessary wear on the machinery, often requiring replacement of parts which results in production delays. External hydraulic pump mechanisms, while being more reliable than disc brakes, have their own attendant problems. An external pump contains more parts and is more complex in structure than one that is integral to the drive head assembly. Thus, an external pump is more liable to failure and more expensive to fabricate than a pump that is integral with the drive head assembly.
Also, as a result of this torsional displacement the sucker rod has a tendency to get longer at either end in much the same way a rubber band does when it is twisted longitudinally. Prior art clamps have not been effective in accommodating this longitudinal movement of the sucker rod/polished rod assembly caused by these torsional forces thus causing decreased pump efficiency. Also, the static clamping mechanisms of prior art inventions do not provide an easy method for lifting the rotor from the stator in order to backwash the hole of sand and grit.
Accordingly, there exists a need for a device for controlling polished rod recoil that contains few moving parts, is of fairly simple construction, is integral with the drive head, and is not prone to failure. There is also a need for a clamp for securing a polished rod and associated sucker rod to a main shaft of a pump drive-head assembly in a manner that allows for some longitudinal freedom of movement, so as to account for longitudinal expansion from torsional forces on the sucker rod, and so as to also provide an easy method for backwashing the hole.